Moisture sensitive element

ABSTRACT

A moisture sensitive element formed of an oxide complex semiconductor comprising 89.9 to 20 mol% of ZnO, 0.1 to 20 mol% of Cr2O3 and 10 to 60 mol% of at least one member selected from oxides of certain kinds of mono-, di-, tetra-, penta- and hexavalent metals.

United States Patent 1191 Ichinose et al.

[ Dec. 16, 1975 MOISTURE SENSITIVE ELEMENT ['75] Inventors: Noboru lchinose, Yokohama; Yuhji Yokomizo; Masaki Katsura, both of Tokyo, all of Japan 22 Filed: July 11, 1974 21 Appl. No.: 487,533

[30] Foreign Application Priority Data [58] Field of Search 252/517-521; 338/20 [56] References Cited UNITED STATES PATENTS 3,766,098 10/1973 Masuyama et al .1 252/519 Primary Examiner-Benjamin R. Padgett Assistant ExaminerE. Suzanne Parr Attorney, Agent, or FirmStewart and Kolasch, Ltd.

July 13, 1973 Japan 48-78476 July 1973 Japan 48-78477 A moisture sensitive element formed of an oxide com- 1973 Japan 48'78478 plex semiconductor comprising 89.9 to 20 mol% of Sept r r 48405455 ZnO, 0.1 to 20 mol% of c1 0 and 10 to 60 mol% of Sept 1973 Japan 48405456 at least one member selected from oxides of certain h l t- 521 US. (:1. 252/517; 252/518; 252/519; :3 Ofmono tetra penta and em me 252/520; 252/521 [51] Int. Cl. HOIB l/Ofl 8 Claims, 37 Drawing Figures LLI O I I 41 0 5 1o 15 2 FIG.

FIG. 2

8 6 4 w m w cr o (MOL RESISTIVITY (Q) RESISTIVITY (Q) US. Patent Dec. 16,1975 Sheet30f 12 3,926,858

FIG. 5

RESISTIVITY (Q) O 5 1O 15 2O CF203 (MOL US. Patent Dec. 16, 1975 RESISTIVITY (Q) RESISTIVITY (.O.)

RELATIVE HUMIDlTYI o) FIG. 21

RELATIVE HUMIDITY(%) RESISTIVITYLQ) RESISTIVITYUU RELATIVE HUMIDITY FIG. 22

RELATIVE HUMIDI TY(%) IOO US. Patent Dec. 16, 1975 Sheet 10 of 12 3,926,858

40 6O TEMPERATURE (C) TEMPERATURE (c) MOISTURE SENSITIVE ELEMENT This invention relates to a moisture sensitive element which detects the variation of humidity with electric resistivity. and more particularly to an element formed of an oxide complex semiconductor.

A prior art moisture sensitive element comprises a fine powder of a metal oxide such as Fe O Fe O A1 0,, and Cr O which is coated on an inorganic insulating substrate. This sensitive element is based on excellent hygroscopicity which the metal oxide generally has. The coated powder of the metal oxide exhibits significant variation in electric resistivity in response to the variation in ambient humidity. Further, a moisture sensitive element utilizing metal oxide film is known. This element has small weight and size, and promptly responds to humidity changes. In addition it satisfactorily functions over a wide range of temperatures, from a low to a high temperature.

However, although the element using powdery metal oxide is physically, chemically, and thermally stable, the coated layer of powdery metal oxide generally has high resistivity and therefore the element hardly detects electrically the changes of humidity with accuracy when such changes are small. Moreover, its reproducibility of the measured value and aging property are not satisfactory. The element based on metal oxide film sometimes fails to measure the humidity accurately, and is inferior in sensitivity due to the unstability and nonhomogeneity of the film. In addition the element can hardly be mass produced.

A group of materials called oxide semiconductors which are prepared essentially from metal oxides may be used in a moisture sensitive element since the oxide semiconductor has lower resistivity than raw metal oxide and is therefore assumed to show great changes in resistivity due to the absorption and release of moisture. However, most of the oxide semiconductors have, as they are called thermistors (i.e., thermally sensitive resistors), a great negative temperature coefficient of resistivity and their resistivities are much affected by only a little change of temperature. The utilization of the oxide semiconductor has therefore been neglected in measurement of humidity accompanied with variation of temperature.

It is an object of the invention to provide a moisture sensitive element suffering little from' aging and having a high sensitivity, a high reproducibility of the measured value, and a small temperature coefficient of resistivity.

Another object of the invention is to provide a moisture sensitive element formed of an oxide complex semiconductor.

In accordance with this invention there is provided a moisture sensitive element formed of an oxide complex semiconductor comprising 89.9 to 20 mol% of ZnO, 0.1 to 20 mol% of Cr O- and 10 to 60 mol% of at least one of the third metal oxides selected from the group consisting of Li O, Na O, K 0, Rb. O, Cu O. BaO. SrO, CaO, PbO, MnO, NiO, CoO, MgO, CdO, CuO, FeO, BeO, TiO. GeO ZrO MnO TeO SnO SiO CeO ThO HfO Nb O Ta O Sb O V W0 M00 and TeO This invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

LII

FIG. I shows the content of Cr O and the resistivity of ZnO-Me OCr O;, system in which the molar ratio of Me- O to ZnO is rendered constant:

FIG. 2 shows the content of Cr O and the resistivity of ZnO-Me O-Cr O system in which the molar ratio of Me O to ZnO is rendered constant;

FIG. 3 shows the content of Cr O and the resistivity of ZnO-Me""O -Cr O system in which the molar ratio of Me O to ZnO is rendered constant;

FIG. 4 shows the content of Cr- ,O and the resistivity of ZnO-Me O -Cr O system in which the molar ratio of Me- O to ZnO is rendered constant;

FIG. 5 shows the content of Cr O and the resistivity of ZnO-Me Q -Cr O system in which the molar ratio of Me O to ZnO is rendered constant;

FIG. 6 shows a plan view of a humidity measuring device using a moisture sensitive element according to this invention;

FIG. 7 shows the relationship between the humidity and the resistivity of a prior art moisture sensitive element at a constant temperature;

FIGS. 8 to 22 show the relationship between the humidity and the resistivity of the moisture sensitive elements according to this invention at a constant temperature; and

FIGS. 23 to 37 show the relationship between the temperature and the resistivity of the moisture sensitive elements according to this invention at a constant humidity.

This invention is based on the discovery that even an oxide semiconductor, though classified as a thermistor, turns out a moisture sensitive element having a low resistivity and a small temperature coefficient of resistivity if its composition ratio is defined as above. Such an element exhibits a resistivity greatly changing with the humidity variation and being little affected by ambient temperature.

The oxide semiconductor forming a moisture sensitive element of the invention contains, in addition to ZnO and Cr O at least one oxide of certain mono-, di-, tetra-, pentaand hexavalent metals (the oxide being hereinafter referred to as the third metal oxide). The oxide (Me. ,O) of monovalent metals of the third metal oxide is selected from Li. ,O, Na O, K 0, Rb O and Cu O, that (Me O) of divalent metals from BaO, SrO, CaO, PbO, MnO, NiO, CoO, MgO, CdO, CuO. FeO and BeO, that (Me o of tetravalent metals from TiO GeO ZrO MnO TeO- SnO SiO CeO ThO and HfO that (Me O of pentavalent metals from Nb- O Ta O Sb O and V 0 and that (Me O of hexavalent metals from W0 M00 and TeO Any mixture of the third metal oxide may be used in the invention.

The content of ZnO is 89.9 to 20 mol%, preferably to 30 mol%, that of Cr O .O.l to 20 mol%, preferably 5 to 20 mol%, and that of the third metal oxide 10 to 60 mol%, preferably 15 to 50 mol%. When the content of Cr- O is less than 0.1 mol% and/or when that of the third metal oxide is less than 10 mol%, the resultant element has a resistivity as large as M9, and is a poor moisture sensitive element. This is true also when the content of ZnO is less than 20 mol% and/or when that of the third metal oxide exceeds 60 mol%.

When the content mol%) of Cr O is varied with the molar ratio of the third metal oxide to ZnO fixed at 1:3, the resultant oxide complex semiconductors have the resistivity characteristics as shown in FIGS. 1 to 5. FIG. 1 indicates the resistivity characteristics of the semiconductor containing the third metal oxide Me o, in which curves a, b and represent the resistivity characteristics in the case where Me are Li, K and Cu. respectively. FIG. 2 indicates the resistivity characteristics of the semiconductor containing the third metal oxide Me o, in which curves d, e and f represent the resistivity characteristics in the case where Me are Sr, Ni and Co, respectively. FIG. 3 shows the resistivity characteristics of the semiconductor containing the third metal oxide Me O in which curves g, h and 1' represent the resistivity characteristics in the case where Me are Ti, Sn and Ce, respectively. FIG. 4 shows the resistivity characteristics of the semiconductor containing the third metal oxide Me O in which curves j, k and I represent the resistivity characteristics in the case where Me" are Ta, Sb and V, respectively. FIG. 5 indicates the resistivity characteristics of the semiconductor containing the third metal oxide Me O;, in which curves m, n and 0 represent the resistivity characteristics in the case where Me are W. Mo and Te, respectively.

As these figures indicate, a content of Cr O more than 20 mol% causes the resultant semiconductor to have a resistivity of over 100 MO and to become an unsuitable moisture sensitive element. All the above being considered, it is apparent that the upper limit of ZnO content is 89.9 mol%. Particularly, a mixture of 60 mol% of ZnO, mol% of Cr O and 30 mol% of the third metal oxide can make a highly desirable moisture sensitive element.

The moisture sensitive element of this invention may be prepared in the following manner. Raw metal oxides accurately weighed out in prescribed amounts are mixed together in a ball mill and presintered at a relatively low temperature, for example, at 600 to 900C. The presintered mass is pulverized into powder. The raw materials used may be metal compounds such as a hydroxide, carbonate and oxalate which can be converted to the oxide upon heating.

The powder thus obtained is mixed with a binder such as polyvinyl alcohol, and the mixture is shaped, under a pressure of about I00 to 1,000 kg/cm into, for example, a plate having a width of 10 mm, a length of mm and a thickness of l mm. The plate is sintered at about 1,000 to 1,300C generally in air. During sintering, it is kept at a maximum temperature for l to 5 hours.

The moisture sensitive element prepared as above is constructed in, for example, a humidity measuring device as shown in FIG. 6. In the figure, numeral 1 denotes the moisture sensitive element of the invention, and numerals 2 and 3 electrodes made of material such as a high-temperature baking silver paint which well adheres to and has a little contact resistance with the element 1.

The moisture sensitive element of the invention, though unexpectedly composed of the oxide semiconductor, has its resistivity hardly changed with temperature. Further, it exhibits such an excellent aging property that its resistivity varied within only several percents even it has been used for a long period of time. Its resistivity variation accompanying with its aging is negligibly small as mentioned above. The element is therefore practically satisfactory. Furthermore. it responds to humidity variation far morequickly than a prior art element. Its sensitivity or responding speed is such that it responds in about 10 seconds to O-to-l00 percent humidity change, and in 2 seconds after a humidity change of 3:20 percent. A prior art element responds in about 40 seconds and in 5 to 7 seconds to O-to-lOO percent and 'i20 percent humidity changes, respectively. With respect to the element of the invention no or little difference occurs between the plotted value of resistivity measured by a decreasing humidity and that measured conversely by increasing humidity. In addition the element of the invention can be manufactured economically since the raw materials used are obtained cheeply.

It has not yet been fully understood why the moisture sensitive element of the invention shows such an excellent characteristic as described above. However, it is assumed that when water vapor deposits on a surface of the element, the element becomes electrically conductive just as a semiconductor becomes, as is well known, when it adsorbs a gas.

This invention will be more clearly understood with reference to the following Examples.

EXAMPLES Mixtures of to 18 mol% of ZnO, 0 to 22 mol% of Cr O and I0 to 62 mol% of the third metal oxide were weighed out as will be indicated below in the Tables and well mixed in a ball mill. The mixture was presintered at 800C for 1 hour and pulverized into a powder, to which was added polyvinyl alcohol as a binder. The mixture obtained was shaped into a plate 10 mm wide, 20 mm long and 1 mm thick. The plate was sintered for 2 hours at 1,000 kg/cm in an electric furnace kept at l,lO0 to 1,300C. Thus, 174 kinds of plate samples including controls were prepared.

To the plate was ordinarily baked silver electrodes, forming a humidity measuring device. Either an elemental Ag or Ag- O may be used as a starting material for the silver electrodes. Since the sintered mass is stable to temperature, the electrode could be baked over a wide temperature range of, for example, 400 to 800C.

With respect to each sample the resistivity (R (0% RH)) which was measured at 25C and a relative hu midity of 0 percent, and the ratio (R (0% RH)/R RH)) of the resistivity at 25C and a relative humidity of 0 percent to that at 25C and a relative humidity of 100 percent were obtained as shown in Tables 1 to 6. Table 1 shows the results with respect to samples containing the third metal oxide Me O, Table 2 the results with respect to samples containing the third metal oxide Me o, Table 3 the results with respect to samples containing the third metal oxide Me O Table 4 the results with respect to samples containing the third metal oxide Me O Table 5 the results with respect to samples containing the third metal oxide Me O and Table 6 the results with respect to samples containing a mixture of the third metal oxides Me O, Me O, Me O- MeflO and Me o Some of the above-mentioned samples were tested for resistivity characteristics to humidity variation with the temperature kept at 25C. The results are indicated in FIGS. 8 to 22, which show the characteristics of the samples of Examples 6, I3, 23, 57, 60, 78, 90, 100,

H3, H7, I26, 135, 139 and 146, respectively.

As a comparison, the resistivity-humidity characteristics of a prior art element formed of a sintered mixture of silicon and a metal oxide are indicated in FIG. 7. This figure reveals that appreciable difference occurs between resistivity measured by a decreasing humidity and that measured conversely by increasing humidity. On the contrary, the moisture sensitive element of the invention gives rise to, as is apparent from FIGS. 8 to 22. no or only a little of such a difference, indicating that it reproduces the measured value much more highly than the prior art element.

Moreover, the resistivity characteristics to temperature variations at relative humidities of 0 and 100 percent were measured with respect to some of the samples. The samples show such characteristics as illustrated in FIGS. 23 to 37. FIGS. 23 to 37 show the resistivity characteristics of the samples of Examples 3. l5. 25.39.50.616, 74. 91. 102. 111, 116. l26. 132, I37 and 147. respectively. These figures clearly show that the moisture sensitive element of the invention. though comprised of the oxide semiconductor. indicates a resistivity which changes little with temperature varia- Table 1 Zn() Cr Q, Me '0 R27, R- .,-,1 (1"; RH 1/ (mol'lr) (mol?) lmol z) ((V'PRH) R. -,1IUU"1RH) (Mil) Control l '90 Me =Li 10 I55 I75 "2 'l =Na1 l7(l I90 Example I 89.9 0.1 =Li 74 785 2 =Na 3 s10 3 =K 57 745 4 =Rh 6] S 5 =Cu 78 710 6 80 5.0 =Li l5 l0 l64(l 7 =Na 13 1x55 s 17 1705 1 =Rh 11 30110 l0 =Cu 24 1570 Example ll 80 5.0 Mc =Na 3 ll] ZIZU H :K I, =Rh ,1

. =Li ll 60 ll) =Li 3H 0,50 8090 13 =Na 0.26 10770 l4 =K 0.34 9565 l5 =Rh (1.47 H l6 =Cu 0.63 7940 l7 =Li l() (1.38 9675 =Rh .7 =Cu I9 30 2O =Li 5U 28 16115 2O =Na 34 l4l0 ll =K 1300 Z2 =Rh 46 N75 23 =Cu 22 1085 24 =Li '10 20 172i) ,1 =Na =K =Rh H H :01 "5 2i) =Li 845 26 =Na 7X 8H) Example 27 20 20 Me =K 6Q 83 760 Z8 =Rb X7 720 29 =Cu 65 105 30 =Li 30 69 885 =K 31 =Rb 62 920 n =Cu Control 3 l 8 2O =K 62 I56 180 4 Z2 =Cu 6U BIS 145 Table 2 ZnO Cr O Me' O R R 0% RH (mol /1) (moV/z) (mol7r) (0 eRH) R- IOOQZRH) Control 5 90 0 Me"- =Ba 10 200 I85 6 ==Ca l5() l9() Example 32 89.9 0.1 =Ba 545 33 =Sr 82 590 34 =Ca 66 640 35 =Pb 87 735 36 =Mn 73 560 37 =Ni 61 705 38 =Co 58 680 I 39 =Mg 60 770 40 ==Cd 69 710 4l =Cu 74 665 42 =Fe 77 e135 3 =Be 85 520 44 5.0 =Bu 15.0 31 I435 Example 45 80 5.0 M =Ca l5.() 28 1620 46 ==Pb ll l9l5 Table 6-continued Example 170 0.5 7

What we claim is:

l. A moisture sensitive element formed of an oxide complex semiconductor comprising 89.9 to mol% of ZnO 0.1 to 20 mol% of Cr. ,O;, and 10 to 60 mol% of at least one of the third metal oxides selected from the group consisting of Li O, Na O, K 0, Rb O, Cu- O, BaO, SrO, CaO, PbO, MnO, NiO, C00, MgO. CdO, CuO, FeO. BeO, TiO GeO- ZrO MnO- TeO- SnO SiO CeO ThO HfO Nb O Ta O Sb O V 0 W0 M00 and TeO 2. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the monovalent metal selected from the group consisting of Li- O, Na O. K 0, Rb O, C1120 and mixtures thereof.

3. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the divalent metal selected from the group consisting of BaO, SrO, CaO, PbO. MnO, NiO, CoO, MgO, CdO, CuO, FeO, BeO and mixtures thereof.

4. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the tetravalent metal selected from the group consisting of TiO GeO t ZrO MnO TeO- SnO SiOg, CeO ThO- HfO- and mixtures thereof.

5. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the pentavalent metal selected from the group consisting of Nb O Ta O Sb O,-,. V 0 and mixtures thereof.

6. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the hexavalent metal selected from the group consisting of W0 M00 TeO and mixtures thereof.

7. A moisture sensitive element according to claim 1 wherein said semiconductor comprises to 30 mol% of ZnO, 5 to 20 mol% of Cr. ,O and l5 to 50 mol% of the third metal oxide.

8. A moisture sensitive element according to claim 7 wherein said semiconductor comprises 60 mol% of ZnO, 10 mol% ofCr O and 30 mol% of the third metal oxide. 

1. A MOISTURE SENSITIVE ELEMENT FORMED OF AN OXIDE COMPLEX SEMICONDUCTOR COMPRISIG 89.9 TO 20 MOL% OF ZNO, 0.1 TO 20 MOL% OF CR2O3 AND 10 TO 60 MOL% OF AT LEAST ONE OF THE THIRD METAL OXIDES SELECTED FROM THE GROUP CONSISTING OF LI2O, NA2O, K2O, RB2O, CU2O, BAO, SRO, CAO, PBO, MNO, NIO, COO, MGO, CDO, CUO, FEO, BEO, TIO2, GEO2, ZRO2, MNO2, TEO2, SNO2, SIO2, CEO2, THO2, HFO2, NB2O5, TA2O5, SB2O5, V2O5, WO3, MOO3 AND TEO3.
 2. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the monovalent metal selected from the group consisting of Li2O, Na2O, K2O, Rb2O, Cu2O and mixtures thereof.
 3. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the divalent metal selected from the group consisting of BaO, SrO, CaO, PbO, MnO, NiO, CoO, MgO, CdO, CuO, FeO, BeO and mixtures thereof.
 4. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the tetravalent metal selected from the group consisting of TiO2, GeO2, ZrO2, MnO2, TeO2, SnO2, SiO2, CeO2ThO2, HfO2 and mixtures thereof.
 5. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the pentavalent metal selected from the group consisting of Nb2O5, Ta2O5, Sb2O5, V2O5 and mixtures thereof.
 6. A moisture sensitive element according to claim 1 wherein the third metal oxide is the oxide of the hexavalent metal selected from the group consisting of WO3, MoO3, TeO3 and mixtures thereof.
 7. A moisture sensitive element according to claim 1 wherein said semiconductor comprises 80 to 30 mol% of ZnO, 5 to 20 mol% of Cr2O3 and 15 to 50 mol% of the third metal oxide.
 8. A moisture sensitive element according to claim 7 wherein said semiconductor comprises 60 mol% of ZnO, 10 mol% of Cr2O3 and 30 mol% of the third metal oxide. 